LCA Tejas: News and Discussions

Sid · Post by **Sid** » 24 Jan 2016 10:41

What Shiv asked is not an unreasonable question, he is laying down some variables. If you are learned enough lay down the equation and use some assumptions.

~20 AoA can be assumed safely as AoA for an air-show, specially for a first timer like LCA. If calculations falls below or above then we know there is some problem in variables.

Alternatively you can also look at tilt of the angle of AoA sensor.

member_28108 · Post by **member_28108** » 24 Jan 2016 10:45

The angle of attack is relative to the flow of air which is unperturbed by the airfoil and not the horizontal. Shiv wants to approximate the airflow by using the vapour trail as a marker. One additional problem is that the vapour trail will be in the immediate area of turbulence adding another variable to the estimate. This will add to the error in estimation.
it is a nice problem and this is how extrapolation with partial data is done a thing that the hman brain is particularly adept at and ia research focus in fuzzy logic and neural networks.

Sid · Post by **Sid** » 24 Jan 2016 11:29

Not an expert but vapor trail will be disturbed by vortex generated by wings at low speed, which should be quite visible.

See below

Karan M · Post by **Karan M** » 24 Jan 2016 11:31

lol, shiv you have set a cat amongst the pigeons. good fun and interesting posts.

tsarkar · Post by **tsarkar** » 24 Jan 2016 12:16

shiv wrote:Anyone want to try this?

Taking a break from the INS Arihant thread,

While other's have answered this, my take is that doing analysis of flight characteristics using photographs without references is not a scientific approach.

Example of unscientific approach is the Internet Forum analysis of structural distortion in ships known as "Hungry Horse phenomenon".

Quality of steel has significantly improved over in the last few decades. Older battleships of used steel many inches or even feet thick, for both structural strength (to carry & fire 14"-16" cannons) and armour protection. They gave a visual impression of solid look.

Modern grades of steel offer better strength and are a fraction of inches thick. Soviets used 5 mm thick steel for Northern Waters where corrosion is less, UK used 8 mm and India uses 10 mm (Page . All are less than an inch (25.4 mm). The edges of the hull plates are welded to the structural frames. This causes a compound curve in the hull plates.

The curved plates, when visual assessed by Internet Forum analysts, given a mistaken assumption of the poor structural strength and shipbuilding quality.

Nothing is further from the truth. The strength of a ship comes from its structural frame and not hull plates.

WW2 battleships like HMS Hood, Prince of Wales, IJN Musahi & Yamato sank very quickly because even though hull plates were inches and feet thick, the structural frame was not as relatively strong. The thousands of tonnage of steel on the hull did not translate into structural strength.

Modern warship building, unless built to commercial or lower standards to save costs, use significantly improved structures, learnt from experience.

So in a nutsheel, visual analysis is extremely unscientific and misleading.

Coming to your photograph of Tejas. The aircraft is flying in three dimensions. To accurately assess any angle using Cartesian Geometry, one needs reference points.

To map an object in three dimensions into two, there is a field of study called orthographic projection. In my time, this was taught in first year engineering. There will be elaborate details on the science of orthographic projection on the internet and Prasanna has accurately described it in his post.

Unfortunately, using a contrail (that itself is in 3 dimensions) as reference is not accurate. And there is nothing in the photo that gives a reference point based on which an orthographic projection can be made.

Secondly, as Siva pointed out, the angle of the plane's longitudinal axis to the horizontal is pitch, not AoA which is the angle between the relative wind & wing. The relative wind is in three dimensions. The photograph gives no indication of the relative wind.

Sid's analysis of the AoA indicator assumes his horizontal x axis of the relative wind to be parallel to the ground. Maybe, maybe not. Also, the aircraft is using its elevons during takeoff to pitch up, so that needs to be accounted for as well.

Hence calculating the AoA based on the photograph is not meaningful, because of lack of our ability to project it in two dimensions and calculate it relative to the wind.

As a aside, let me touch upon some very real tribal science for angle of attack measurement. I'm a sailor for four decades.

Most people incorrectly assume that a boat moves when wind pushes against the sail. So a boat is dependent on the wind from rear to push it. If the wind comes from any other direction, then the boat will be unable to travel in the desired direction. Unfortunately, this is not correct.

Irrespective of the direction of the wind, the yacht can go in any direction from zero to 360. How?

By adjusting the angle of the sail to the wind. By adjusting the angle of the sail to direction of the wind, a boat can travel in any direction. Its called points of sail http://www.rya.org.uk/SiteCollectionIma ... il-lrg.jpg

The sail of the boat is the first wing using wind for lift invented millennia ago before the aircraft.

This knowledge enabled the Europeans to sail across the world. When the Zamorin of Calicut along with his Arab allies put together a fleet, the Portuguese outsailed them with the knowledge of adjusting the wind to the sail.

Most people wrongly attribute European victories at sea to cannons. The victories were due to their knowledge of sailing. They could adjust the angle of the sail to the wind, and then either escape enemy fire or come in from multiple directions to rake enemies lacking this knowledge and who're sailing uni-directionally with the wind to the back.

There is a motivational saying in the sailing community, "We cannot change the wind, but we can always adjust our sails"

But how do sailor tell the direction of the relative wind to adjust the angle of sails?

They do so roughly using flags & pennants - which is why old paintings of ships show large flags & pennants on every mast. Especially during battles, you'll find large number of flags & pennants flying. The reason is not religious or nationalistic fervor but to quickly ascertain wind direction & wind shifts. Just look at the number of flags here http://www.1zoom.net/big2/785/285023-alexfas01.jpg

But for finer adjustment of sail to relative wind, a simple & ingenious method called tell tales are used.

A tell tale is a piece of wool taped, nailed or tied to the sail and the various ropes or wires (called stays) holding the mast, and give an accurate direction of relative wind based on which the angle of attack of the sails can be finely adjusted. Tell tales are fixed to both the windward and leeward sides of the sail & stays on the boat to show whether a sail is properly adjusted.

Here is a photo of tell tales at work http://www.windcraft.com.au/blog/wp-con ... 5x1024.png

Coming back to aviation. Late Group Captain Kapil Bhargava, among our best test pilots, often praised Air Marshal Johnny Greene as better than him

http://www.bharat-rakshak.com/IAF/Perso ... reene.html

The flight with Johnny was a surprise to me. I had won the Flying Trophy in my course but I could see that Johnny's flying was noticeably superior. On landing I reported and recommend to Dicky that he should fly with Johnny. He promptly asked me why and what was wrong with him. I explained that there was nothing wrong with him, it was just that he flew better than any other pilot or instructor I had ever flown with. Dicky did as I suggested and had the grace to call me and tell me that I was quite right!

And Johnny Greene did something innovative

On another occasion I learnt a valuable P of F trick from Wg Cdr Greene in 66-67. One day I was asked by my Flt Cdr to escort him for a Hunter 56 sortie. Just before leaving the crew room JWG asked me to bring along a roll of scotch tape, which I did unquestioningly (P/Os were neither meant to be seen nor heard!). After doing his external checks he asked an airman for a piece of stocknit (govt issued cleaning rag) from which he very carefully extracted a single thread, about 3" long. He used a short length of the clear cello tape to attach this piece of thread to the side of the sliding canopy, the 'leading' end of the thread being covered and held by the tape. I was most intrigued and, very hesitatingly, asked him what it was for. He gave me a 'you-must-be-pretty-dumb' look. "It's my angle of attack indicator", he said, without further explanation. What a simple but brilliant idea, I thought to myself. Later on I tried the same thing on dozens of sorties on the Hunter and Type77 and found the indications to be very accurate and illustrative. 18 years later while commanding 28 & 30 Sqn (both conducting 3rd Stage training) I used Johnny Greene's AOA indicator on the Type 66 to demonstrate visual indication of the airflow to the young trainees.

Johnny Greene used a tell tale like sailors! He used this knowledge to his competitive advantage.

Johnny Greene was from Colaba, where there are sailing clubs, and I speculate that he possibly learnt it in his youth sailing. Maybe he learnt it elsewhere. But he used science to his advantage.

Now back to the Arihant thread

raghava · Post by **raghava** » 24 Jan 2016 12:41

In the last few weeks, there have been some nice updates collated from the LCA Tejas Facebook page by BRF members - However, these posts are scattered over the last few pages. This is my small attempt to collate these bits of information -

Confirmed Orders - Total confirmed orders for 120 aircraft of which 20 will be in the Mk1 and 100 in the Mk1A configuration

Mk1A - The Mk1A configuration will primarily deal with resolving existing maintenance issues, gun integaration, AESA integration, and will have an integrated EW Suite. Presently the team is waiting for the IAF to specify the kind of BVR integration it wants. The present thinking is that the Mk1A might get the Astra and the NLCA the Derby.

GSh23 Gun - Mk1 will not have operational gun for FOC. The focus of the IAF and the LCA team as of now is on BVR integration.
- However, "butt firing tests and preliminary structural modifications have been completed." Muzzle calibration completed. Vibrations found to be at accepted levels. Suction of gun generated gas into engine not happening on ground. Gas ingestion in flight to be tested. Remaining work to be completed post FOC.

BVR - Integration of Python-5 and Derby on Mk1 ongoing.

Payload - Tejas capable of carrying an external payload of 4.5 tons. (** whether this is due to taking off with reduced fuel followed by aerial refuelling is not yet clear.. more clarity required on clean weight and MTOW)
- 725 litre drop tank separation already cleared. However, the drop tank in combination with other stores like tandem bomb racks yet to be tested.
- Derby and Python can be carried simultaneously both in MB and IB.
- Inboard Pylons for tandem bomb racks are in final stages of integration.
- 1000 lbs mkII unguided bombs, LGBs and PB500 successfully flight tested.

Cobham Radome - Integration and ground tests over. Flight testing to commence soon

IFR Probe - Integration and ground testing ongoing

Ballast - LCA flies with ballast but it is not as "big a number" as 300kg. IFR probe integration will reduce the ballast weight, but by how much exactly will become clear once frozen SOP starts flying.

Weight reduction - Some weight reduction possible but the 800kg figure bandied about in the media is "doubtful"

Undercarriage - There is an issue with the undercarriage and this issue is responsible for the delay in rollout of SP airframes. There is only a temporary fix for now.

AoA and g's - Flight testing completed for 24 deg and cleared for 22 deg. LCA is cleared for and has demonstrated 8g. **In response to a question on beyond 8g, he was vague but mentioned " for 9g, structural simulation has been done though not on the aircraft as a whole"

SPJ - the team is exploring options to integrate the SPJ on OB pylon or on wingtips.

shiv · Post by **shiv** » 24 Jan 2016 12:47

^^^
Raghava. many thanks

Sid · Post by **Sid** » 24 Jan 2016 12:49

Following is a pic of LCA in level flight. Notice the AoA sensor orientation. Can anyone explain what that can mean?

tsarkar · Post by **tsarkar** » 24 Jan 2016 12:53

ramana wrote:
sankum wrote:PB500 bomb tested on tejas
Very good. Means Mk 83 type bombs in addition to HSLD, Russian, and UK origin.

Given our integration experience of sensors & weapons, viz DARIN, DARIN 2 & 3 on Jaguar, Litening on Mirage & Su-30, Derby on Sea Harrier, the sensor & weapons integration & qualification on Tejas has actually been quite fast, and preceded testing, opening & certification of the flight envelope.

Many members believe weapons integration & qualification is good enough for induction.

However, its the testing, opening & certification of the flight envelope is what is/was delaying induction, like IOC2 in December 2013, post which the aircraft was certified to fly without telemetry and the more recent max g testing.

By all indications, the entire Tejas team followed an extremely safe & methodical flight test program. There will be benefits of this investment of time & effort.

There will be great many numbers of Tejas in service beyond the initial 100s. The dwindling number of fighter designs in the world and the perfidy of manufacturers to protect their revenue streams by denying ToT will ensure significant numbers of Tejas in IAF service.

I wish ADA/HAL takes the aircraft, especially two seaters, to stations across India and give fighter pilots a feel of its capabilities. Today, a handful of pilot know its actual capabilities, and most opinion is formed by the negative media reports.

JayS · Post by **JayS** » 24 Jan 2016 13:32

tsarkar wrote:
I wish ADA/HAL takes the aircraft, especially two seaters, to stations across India and give fighter pilots a feel of its capabilities. Today, a handful of pilot know its actual capabilities, and most opinion is formed by the negative media reports.

+1000. Great idea. As the oft-repeated old quote (can't remember whose) on this forum goes - Those who fly LCA, swear by it and those who deride it have never actually flown it.

manjgu · Post by **manjgu** » 24 Jan 2016 15:35

tsarkar..by structure frame u mean the skeleton of the ship on which plates are welded?? any link on this subject ??

what is the importance/advantage of a pilot knowing his planes AoA of his plane in flight? does this not come instinctively to a pilot as to the limits of what a plane can do...

member_28108 · Post by **member_28108** » 24 Jan 2016 15:56

^There is an angle of attack limiter in planes.This has to be actually inactivated to do maneuvers like the Kulbit or Pugachev cobra by the pilot. Getting the Horizon right can be confusing at times for a pilot so AoA , G forces etc need not come "Insitinctively'

SaiK · Post by **SaiK** » 24 Jan 2016 18:46

it would be fantastic to see a kulbit with tejas.

sanjaykumar · Post by **sanjaykumar** » 24 Jan 2016 20:02

Absolutely fabulous post tsarkar. However, using vectors in sailing would be intuitive with experience and not a monopoly of Europeans.

Cain Marko · Post by **Cain Marko** » 24 Jan 2016 20:22

Amazing post tsarkar sahab, great anecdotes too. So would it be possible to figure out aoa if there was a Windsock in the picture?

SaiK · Post by **SaiK** » 24 Jan 2016 21:21

tsarkar ji, thanks.. but one small pooch, what is that 'no-go zone'? does it mean one can't do all 360* direction but say only 270 or you can't do a reverse thrust?

JayS · Post by **JayS** » 24 Jan 2016 21:29

@CM

^^ provided its put on a carefully chosen place on the a/c body where the local flow is aligned to the free stream wind direction in most of the situations. Any random location will not work. However its theoretically possible for an experience pilot to have developed his own mental correlation between the angle shown by such "tell tale" at some specific location and actual AoA. I'd imagine its more intuitive of sort than knowing accurate numerical value.

Will · Post by **Will** » 24 Jan 2016 22:42

With the IAF committed to over a 100 Mk1a there is no pressure on the MKII. Maybe the ADA should just jump to a bigger stealthy MKIII.

Indranil · Post by **Indranil** » 25 Jan 2016 00:11

tsarkar wrote: And Johnny Greene did something innovative

On another occasion I learnt a valuable P of F trick from Wg Cdr Greene in 66-67. One day I was asked by my Flt Cdr to escort him for a Hunter 56 sortie. Just before leaving the crew room JWG asked me to bring along a roll of scotch tape, which I did unquestioningly (P/Os were neither meant to be seen nor heard!). After doing his external checks he asked an airman for a piece of stocknit (govt issued cleaning rag) from which he very carefully extracted a single thread, about 3" long. He used a short length of the clear cello tape to attach this piece of thread to the side of the sliding canopy, the 'leading' end of the thread being covered and held by the tape. I was most intrigued and, very hesitatingly, asked him what it was for. He gave me a 'you-must-be-pretty-dumb' look. "It's my angle of attack indicator", he said, without further explanation. What a simple but brilliant idea, I thought to myself. Later on I tried the same thing on dozens of sorties on the Hunter and Type77 and found the indications to be very accurate and illustrative. 18 years later while commanding 28 & 30 Sqn (both conducting 3rd Stage training) I used Johnny Greene's AOA indicator on the Type 66 to demonstrate visual indication of the airflow to the young trainees.
Johnny Greene used a tell tale like sailors! He used this knowledge to his competitive advantage.

Johnny Greene was from Colaba, where there are sailing clubs, and I speculate that he possibly learnt it in his youth sailing. Maybe he learnt it elsewhere. But he used science to his advantage.

Thank you for the anecdote. This technique is in widespread use in gliders. They are called slip string (when used to check the side slip) and side string (when used to measure the the AoA). These were the very first "aids" the pilots had when sophisticated instruments were not developed yet. If you think about it, the use of "tufts" to visualize airflow around an aeromodel or plane essentially works in the same manner.

Khalsa · Post by **Khalsa** » 25 Jan 2016 00:18

Slightly off topic
This thread and the Arjun thread are my only lurking areas now.
So disappointed with the heel dragging on the Rafael amongst other things by the French and us.

Wishing the Tejas more speed, this will be the Godavri of the Indian Air Force.
That will give the IAF to be more confident than ever before of its own designs.

Bring on Tejas Mk 1, 1A, 1b 2 and then bring on the AMCA.

member_29214 · Post by **member_29214** » 25 Jan 2016 01:03

Off topic:
While we spend billions and wait for Rafale, is it worth trying to make a LCA_XL elongated/scaled up version with twin engines to make a medium weight aircraft, while we wait for the AMCA.. I know this topic has been broached before, but dont remember what the BRF community responses were.

Asit P · Post by **Asit P** » 25 Jan 2016 02:21

Thanks everyone for your kind words!

Personally I don't like pictures with watermark and hence I deliberately do not watermark any pic clicked by me. Tejas is a national pride and my amateurish pics of Tejas are for one and for all. Even if a single child, teen or a detractor develops a fascination for Tejas through my pics, I would believe that my objective has been met. That said, I also have high regards for those who watermark or copyright their pics (Many of them are journalists or photographers who make a living out of it).

Karan M · Post by **Karan M** » 25 Jan 2016 02:45

Kudos Asit!

I had much the same attitude when some of the stuff I wrote was picked up by various "professional" folks. Same intent!

Glad to see you sticking by this in an era where every bit of personal promotion is considered a +.

Karan M · Post by **Karan M** » 25 Jan 2016 02:48

akumarAZ wrote:Off topic:
While we spend billions and wait for Rafale, is it worth trying to make a LCA_XL elongated/scaled up version with twin engines to make a medium weight aircraft, while we wait for the AMCA.. I know this topic has been broached before, but dont remember what the BRF community responses were.

Our huge issue is lack of "excess" flexibility in our resource constrained system to take up new stuff even as exploratory programs as you (quite rightly) IMO suggest.

In an ideal world we would be thinking of LCA Mk1A, Mk2, Mk3 and so forth like the Chinese with J-10, swapping wings, redesigning structures, evaluating whatifs and so forth constantly..

But in the other thread, reality bites. An entire civil program given $50mn and cancelled.

SaiK · Post by **SaiK** » 25 Jan 2016 03:38

write your "Will".. it would take the time to deliver the Mk3 if we don't have the decision before Jaitley saab steps in.

Mk2 specs itself is iffy. As said, we should use the Mk3 program like how DRDO and ISRO do test various modules and LRUs for the next generation needs. That is a logical progression to Rafale-type platform as Mk3, pretty much a twin-engined variant like how they did Mirage 4k.

Let us model after France. It fits exactly our agenda.

Cain Marko · Post by **Cain Marko** » 25 Jan 2016 03:58

Asit P wrote:Thanks everyone for your kind words!

Personally I don't like pictures with watermark and hence I deliberately do not watermark any pic clicked by me. Tejas is a national pride and my amateurish pics of Tejas are for one and for all. Even if a single child, teen or a detractor develops a fascination for Tejas through my pics, I would believe that my objective has been met. That said, I also have high regards for those who watermark or copyright their pics (Many of them are journalists or photographers who make a living out of it).

Absolutely in the best form Asit, ..and in the highest traditions of Bharat...kudos indeed

SriKumar · Post by **SriKumar** » 25 Jan 2016 04:10

shiv wrote:Anyone want to try this?

I have given length, wingspan (from Google) and some angles

ABCD is a plane, a parallelogram. The idea is to find the angle at A. Is it possible with the given data?

Some random comments: This picture clearly illustrates the two-dimensional nature of a photograph vs. the three-dimensional nature of real objects. Any photograph is actually a two-dimensional projection of a three-dimensional object. If you look at the angle between the length of the craft and the width of the craft, it is indeed 90 degrees. This angle as seen in the picture, is actually not that much different, visually, from angle A (or, DAB, as one would write in 8th grade geometry). In fact, DAB is visually larger angle than the angle we know to be 90 degrees. So, it gets a bit iffy. One reason for this is that the 2 angles are in different planes- one is in the plane of the craft and angle A is more in a 'vertical' plane i.e. parallel to the vertical fin.

abhik:
I agree with your explanation (of right-angled triangles, rotated around the height) that an estimate from the picture would be an overestimate relative to the actual angle of attack.

SriKumar · Post by **SriKumar** » 25 Jan 2016 04:33

Some additional random thoughts on the above questions..... trying to recall 9th grade geometry here, plus some youtubing....
There are 2 potential sources of error (and therefore 2 corrections needed), in addition to the smoke getting disrupted by the wake of the craft (about which I have no real idea).
- one is that the craft is not square of the camera, it is to the left of the camera; the second is that it is at an altitude and therefore not level with the camera (i.e. craft is not at ground level). Not sure how to account for the second issue, but for the first issue, one can look at the full video and see where exactly is the craft directly in front of the camera.

Full video is here:
https://www.youtube.com/watch?v=PjceCzHTy2s

If you look from 10:01 to 10:40, the craft goes from left of camera to right of camera. So, a perfect sideview will take place between somewhere these two points. A frame where one can see it in perfect silhouette i.e. the two smoke cannisters line up, will indicate the craft is square of the camera (and remove one source of error- altitude is still an issue but may not be a large source of error). I thnk at 10:23 or 10:24 the craft is square of the camera- if you look at the smoking end of the smoke cannisters (and not the tip). My guess is frame grabs at these instants of time would be more useful.

AT this point, the smoke trails are not clear, but nevertheless quite visible. The disruption of the trail starts behind the cannister, at about 2 cannister-lengths away. At this distance, the smoke starts to 'spread out'. I think it would be fair to draw a line in the middle of this portion of the smoke trail, and compare it with the inclination of a line along the smoke cannister. My feeling is that this part of the smoke trail does not see a full-scale disruption from the wake of the craft. That seems to take place about 1 wing-length behind (my guess) if you look at 7.27s/7.28 (it is actually much clearer in some other videos). So, it might be OK to use frames between 10:23/24. This is my 2 cents worth of armchair aerology, mostly youtubing though.

Asit:
You could potentially put your name (or a 'camera pseudo-nym') in some corner of the picture where the subject is not covered. If someone wants to see other pictures by the picture-taker, they know who to search for.

member_29294 · Post by **member_29294** » 25 Jan 2016 06:36

akumarAZ wrote:Off topic:
While we spend billions and wait for Rafale, is it worth trying to make a LCA_XL elongated/scaled up version with twin engines to make a medium weight aircraft, while we wait for the AMCA.. I know this topic has been broached before, but dont remember what the BRF community responses were.

What is more likely to happen is Rafale gets a large order and MII assembly line adding to several hundred units

Then AMCA gets re-purposed and enlarged to replace the PakFA, which would not be inducted, as a 5th generation A2A fighter replacement for Su-30MKI. This would require an engine more powerful than the GE-F414 though, which might be why there is talks of joint venture with GTRE and a foreign partner. This would create the larger engine in the +140-150KN thrust class and put the re-designed AMCA on par with J-20 and PakFA in size, payload, and range. This would be finished in early 2030s if all goes accordingly. Thus making the primary fighter makeup of IAF: Tejas Mk2/Rafale/AMCA ("Heavy") by mid 2030s.

Or maybe I am just crazy. Tejas Mk2 will be finished either way, if only for IN, and then IAF will come after their years of non-support for the program and demand an Airforce version. Just like how they have demanded Raffles. AMCA continues development as a 5th generation medium strike aircraft for induction ~2030, and PakFA gets the MKI treatment to become FGFA and is inducted ~2025. Thus making the primary fighter makeup of IAF: Tejas Mk2/AMCA/FGFA by early 2030s.

NRao · Post by **NRao** » 25 Jan 2016 06:44

is it worth trying to make a LCA_XL elongated/scaled up version with twin engines

That essentially was the original MCA (out of which came the AMCA) without the two fins. It was expected to have TVC to control all the gyrations.

Below is a supposed(?) concept of a 6th Gen plane in the US:

A Sharma · Post by **A Sharma** » 25 Jan 2016 07:23

Sorry if posted before

Pics from DRDO website

Indranil · Post by **Indranil** » 25 Jan 2016 08:05

People, please stop calculating the AoA from that picture. Especially those who are calculating the same by using the contrail of the vortices as the direction of air incident on the wing. Nilesh has described how the vortices behave. This is what it looks like.

The LCA FB admin told you that it is 22 degrees. Go with that.

shiv · Post by **shiv** » 25 Jan 2016 08:20

indranilroy wrote: The LCA FB admin told you that it is 22 degrees. Go with that.

Question is: Is it flying at 22 degrees in that image? How would you find out. It is a math exercise. That's all.

Here is exactly the same thing done with your image. How bad is that approximation of AoA? Stop being a wet blanket

Indranil · Post by **Indranil** » 25 Jan 2016 08:46

The green lines that you used to get that 26.5 degrees are the ones that flow around the wing root and fuselage area. The airflow around that region is like the airflow around the upper surface an airfoil. But the airflow that becomes part of the vortices (the one that you see in that picture) leaves the upper surface of the wing in an upward and outward spiral as shown in the front, top and isometric view. The smoke produced by a generator slung in the outermost pylon of the wing becomes part of these vortices.

I am not trying to be a wet blanket. I am stopping people from going down a wrong path. It is the same as saying, unlike what you observe, the sun doesn't orbit the earth.

The only sane proposition that I have read is that can we find the AoA by checking the position of the AoA vane. Nothing else in the last few pages is a mathematical exercise.

shiv · Post by **shiv** » 25 Jan 2016 09:13

indranilroy wrote: I am not trying to be a wet blanket. I am stopping people from going down a wrong path. It is the same as saying, unlike what you observe, the sun doesn't orbit the earth.

The only sane proposition that I have read is that can we find the AoA by checking the position of the AoA vane. Nothing else in the last few pages is a mathematical exercise.

Indranilroy you have not actually shown why it is not possible to calculate the AoA approximately. All you have done is state opinions. I personally do not find your post convincing and am happy to continue "down the wrong path". Surely you don't believe that learning means always going down one path and not finding out where things can go wrong? In this day and age, with the internet anyone can get any information including stuff like "FB admin says it is 22 deg. Go by that". Sorry. That is not the point. The point is to try and show that the figure obtained from a calculation based on the image can never be right. Can you do that. Yes or no. No need to say anything if you are not interested. I am though and will keep exercising my mind and sharing with others things that fascinate me. Stopping that by saying "wrong path" as if I am trying to mislead and you are not is unnecessary. You could simply keep off the discussion.

Indranil · Post by **Indranil** » 25 Jan 2016 09:39

You are trying to calculate the incidence of air on the wing based on the airflow behind the wing. That too for a region of flight where most of the lift is vortex generated. Without any other reference it is impossible to predict the velocity vector of the C.G. of the plane. If you can't do that you can't calculate the AoA.

And there is no opinion in that. Nilesh and Tsarkar tried to tell you that in words. I provided you with CFD visualizations hoping that you would be able to "see" it. Unfortunately sir, you think you are doing a "mathematical exercise", but all you are doing is being dogmatic. People are trying to tell you where you are wrong, but you won't budge! What you are doing flies in the name of physics, as we know it today. What's worse, you are leading others along the same path with you. At some point it will have to be stopped. At least here.

shiv · Post by **shiv** » 25 Jan 2016 09:45

On the general question of whether a smoke trail is indicative of direction of air flow, the image below, of oil fires in Kuwait should show which of the numbered lines best represents wind direction, vortices, turbulence and all.

The question is why the same principle cannot be used in the case of an aircraft to get a rough estimate of AoA?

<indranilroy: REPLICATING THE POST THAT GOT DELETED BY MISTAKE>

indranilroy wrote:You are trying to calculate the incidence of air on the wing based on the airflow behind the wing.

Please explain how the direction and angle indicated by a trailing string is different from trailing smoke?

indranilroy wrote: Thank you for the anecdote. This technique is in widespread use in gliders. They are called slip string (when used to check the side slip) and side string (when used to measure the the AoA). These were the very first "aids" the pilots had when sophisticated instruments were not developed yet. If you think about it, the use of "tufts" to visualize airflow around an aeromodel or plane essentially works in the same manner.

Indranil · Post by **Indranil** » 25 Jan 2016 10:11

The question is what is the angle of incidence made by the wind on the object.

1. In the above picture, the object is not changing the velocity of the wind (significantly) relative to itself. Therefore by studying the the angle emergence (depicted by the smoke trail), you can infer the angle of incidence.
2. In the case of the aircraft, the object changes the velocity of the wind relative to itself. If you knew the function to calculate this change of direction, then you could use the inverse function on the angle of emergence (depicted by the smoke trail) to calculate the angle of incidence. Otherwise, not.

Indranil · Post by **Indranil** » 25 Jan 2016 10:32

shiv wrote:
indranilroy wrote:You are trying to calculate the incidence of air on the wing based on the airflow behind the wing.
Please explain how the direction and angle indicated by a trailing string is different from trailing smoke?
indranilroy wrote: Thank you for the anecdote. This technique is in widespread use in gliders. They are called slip string (when used to check the side slip) and side string (when used to measure the the AoA). These were the very first "aids" the pilots had when sophisticated instruments were not developed yet. If you think about it, the use of "tufts" to visualize airflow around an aeromodel or plane essentially works in the same manner.

The tuft has to be placed at a location where the air incident air is the predominant flow. Otherwise, it will be useless, unless you know how the incident airflow is being modified by the aircraft(again something that Nilesh tried to explain earlier). That is why a test specimen is covered with many tufts. Each tuft depicts the local airflow around the plane, and these directions may be different from one another. By just looking at the direction of the tuft near the wing tip, you can't infer the direction of the tuft at the wingroot, unless you knew a function of correlation. Similarly, you could not infer the direction of the tuft at the leading edge by looking at the tuft at the trailing edge.

P.S. Hakim, I am sorry, I deleted one of your posts by mistake. It was a series of errors starting with pressing the "Edit" button instead of the "quote" button. Thankfully, the post was captured in its entirety as a quote in this post. I will append it to your last post, so that you may edit the same, if you wanted to. Once again sorry for the mistake and ensuing confusion.

Raman · Post by **Raman** » 25 Jan 2016 10:54

indranil is absolutely right.

Angle-of-attack is measured between the datum line of the wing and the external free stream flow. However, the aircraft (wing and all) modify the flow locally. Specifically, there is significant upwash near the leading edge and even more significant downwash near the trailing edge. Furthermore, there are edge vortices that muddle the matter even more. One cannot simply measure angles from within these regions and expect the result to be valid with respect to the external flow.

In the following diagram the red arrow is the local flow vector at the trailing edge, and the white is the correct external free stream reference vector.

Note that this is a 2D diagram and completely ignores vortices.

For similar reasons, one can't just plonk down an air data probe wherever one wants on an airplane. It has to be chosen in a location where the local flow vector is the same as the external free stream flow. As such, I'm a little skeptical of sticking a tuft at any location on the airplane and expect a reasonable AoA approximation unless the local flow is well understood. It works great for approximating slip angles however.